Slitherine

I notice that in some scenarios where naval forces are present , those slots are counted against the number of auxilary slots I can deploy, even more in Brigthon scenario I have negative slots ( I can´t deploy a single auxiliary unit ) . Is that intentional ?

Probably. IIRC SCW is the only campaign where you get free auxiliary slots that stack with those from Auxiliary Force, but even so the trait is so overpowered it's definitely worth it. As for Brighton, if you're leveraging the trait it's better to disband a few ships, because some -- like the Graf Zeppelin -- are just not worth the slots.

If you are farming prestige with the Power 4, then the expanded AUX trait is probably the most powerful 5th pick in the game.

Don't forget that even if you have a load of AUX units taking up your slots, when you start losing them, you can always summon additional AUX stuff mid battle, at any supply hex.

I often throw aux fighters at the enemy, as they are cheap, can scout for me and once shot down they can be easily replaced and be on the front line very quickly.

more aux slots is definitely powerful in SCW but dont think its needed elsewhere unless as a safety net. Always felt it should be restricted to less powerful german troops as a sort of mock garrison troops , older equipment, maybe captured equipment and troops from allied nations. The core campagin does this few times in barborosaa from memory.

Thanks to everybody , all your answers had helped me a lot.